docs: def _combined_services(cls):  # pylint: disable=no-self-argument
          """
          A dictionary that collects all _services_requested by all ancestors of this XBlock class.
          """
          # The class declares what services it desires. To deal with subclasses,
          # especially mixins, properly, we have to walk up the inheritance
          # hierarchy, and combine all the declared services into one dictionary.
          combined = {}
          for parent in reversed(cls.mro()):
              combined.update(getattr(parent, "_services_requested", {}))
          return combined